In an image pickup device for taking out electric charges corresponding to a received-light amount as a received-light output, as an exposure time becomes longer, the received-light amount increases. In general, an upper limit of the received-light output reflecting the received-light amount is determined depending on the size and the impurity concentration of the charge generating portion or the charge transfer portion. Therefore, even if the exposure time is extended, the received-light output accurately reflecting the received-light amount cannot be obtained when these portions are saturated. Thus, the extension of the exposure time does not always bring about an improvement in sensitivity.
For example, Japanese Patent Early Publication No. 2006-84430 discloses a distance image sensor, which comprises a light emission source for irradiating an intensity-modulated light into a target space, a light detecting element having a plurality of photosensitive portions for receiving light from the target space and generating electric charges corresponding to the received-light amount, a control circuit for controlling a light receiving period where the photosensitive portions receive the light from the target space, and a image generator for determining a distance to an object in the target space by use of electric charges generated by the photosensitive portions, and generating a distance image. In this image sensor, the control circuit controls the timing of transferring electric charges such that the light detecting element provides the electric charges collected for each of detection periods having different lengths to the image generator. In addition, the image generator selects an appropriate detection period from the detection periods having different lengths such that the electric charges collected for the appropriate detection period becomes maximum within a range not exceeding a permissible charge amount of the light detecting element, and determines the distance by use of the electric charges collected for the appropriate detection period.
Thus, the received-light output can be accurately obtained by preventing the photosensitive portions from saturation. However, since the received-light amount is adjusted by changing the length of the exposure time, it is impossible to obtain the electric charge amount more than the permissible charge amount of the light detecting element. In other words, to expand the dynamic range of the received-light output, for example, it is needed to increase the area of the photosensitive portions. However, it leads to an increase in size of the light detecting element.